Interference filter for non-zero angle of incidence spectroscopy

ABSTRACT

The present disclosure relates to thin film optical interference filters. The filters include a substrate and a plurality of alternating material layers deposited on the substrate. When operated at about 45° angle of incidence, the filters exhibit at least one of improved polarization splitting, edge steepness, bandpass bandwidth, and blocking, relative to conventional thin film interference filters.

This application is a division of U.S. patent application Ser. No. 12/129,534, filed on May 29, 2008, entitled “Interference Filter for Non-Zero Angle of Incidence Spectroscopy,” which claims priority to U.S. Provisional Application No. 60/940,701, filed May 30, 2007, the contents of which are incorporated herein by reference.

The present disclosure relates to optical thin-film interference filters, including filters suitable for use in non-zero angle of incidence spectroscopy. The present disclosure also relates to spectroscopy systems including such filters, and methods for making such filters.

BACKGROUND OF THE DISCLOSURE

Thin film interference filters are important components in systems for optical measurement and analysis, such as Raman spectroscopy and fluorescence microscopy. In particular, thin film interference filters, such as optical edge filters, notch filters, and/or laser line filters (LLF's), are advantageously used in such systems to block unwanted light that would otherwise constitute or generate spurious optical signals and swamp the signals to be detected and analyzed. Thus, failure or inadequate performance of these filters can be fatal to operation of a system in which they are utilized.

In general, interference filters are wavelength-selective by virtue of the interference effects that take place between incident and reflected waves at boundaries between materials having different refractive indices. This interference effect is exploited in interference filters, which typically include a dielectric stack composed of multiple alternating layers of two or more dielectric materials having different refractive indices. In the case of a filter which substantially reflects at least one band of wavelengths and substantially transmits at least a second band of wavelengths immediately adjacent to the first band, such that the filter enables separation of the two bands of wavelengths by redirecting the reflected band, the resulting filter is called a “dichroic beamsplitter,” or simply a “dichroic” filter.

In a typical interference filter, each of the respective layers of the filter stack is very thin, e.g., having an optical thickness (physical thickness times the refractive index of the layer) on the order of a quarter wavelength of light. These layers may be deposited on one or more substrates (e.g., a glass substrate) and in various configurations to provide one or more of long-wave-pass (also called long-pass), short-wave-pass (also called short-pass), band-pass, or band-rejection filter characteristics.

In the case of prior known edge filters, the filter is configured so as to exhibit a spectrum having a clearly defined edge, wherein unwanted light having wavelengths above or, alternatively, below a chosen “transition” wavelength λ_(T) is blocked, whereas desired light is transmitted on the opposite side of λ_(T). Edge filters which transmit optical wavelengths longer than λ_(T) are called long-wave-pass (LWP) filters, and those that transmit wavelengths shorter than λ_(T) are short-wave-pass (SWP) filters.

FIGS. 1A and 1B schematically illustrate the spectral transmission of idealized LWP and SWP filters, respectively. As shown in FIG. 1A, an idealized LWP filter blocks light with wavelengths below λ_(T), and transmits wavelengths above λ_(T). Conversely, as shown in FIG. 1B, an idealized SWP filter transmits light with wavelengths below λ_(T), and blocks light with wavelength above λ_(T).

Edge steepness and the relative amount of transmitted light are important parameters in many filter applications. As shown in FIGS. 1A and 1B, an idealized edge filter has a precise wavelength edge represented by a vertical line at λ_(T). As such, an idealized filter has an “edge steepness” (i.e. a change in wavelength over a defined range of transmission) of 0 at λ_(T). However, real edge filters change from blocking to transmission over a small but non-zero range of wavelengths, with increasing values of edge steepness reflecting an edge that is increasingly less steep. The transition of a real edge filter is therefore more accurately represented by a non-vertical but steeply sloped line at or near λ_(T). Similarly, while an ideal edge filter transmits all light in the transmission region (transmission T=1), real filters have some amount of transmission loss, invariably blocking a small portion of the light to be transmitted (T<1).

As a result, the reported edge steepness of a real edge filter depends on the transmission range over which it is defined. Further, as will be discussed below, conventional edge filters exhibit polarization splitting when operated at a non-zero angle of incidence, in which case the corresponding spectra for s and p-polarized light may not have the same edge steepness.

Edge filters, notch filters, and laser line filters are particularly useful in optical measurement and analysis systems that use light from a light source, such as a laser, to excite/illuminate a sample at one wavelength λ_(L) (or a small band of wavelengths) and measure or view an optical response of the excited sample at other wavelengths. The excitation light λ_(L) is delivered to the sample by an excitation light path, and the optical response of the sample is delivered to the eye or measuring instrument by a collection path. Notch filters are generally specialized implementations of edge filters, in that they exhibit a long wave edge and a short wave edge bordering a narrow region of low transmission. Laser line filters are generally configured so as to transmit as much light from a desired wavelength as possible, while blocking other wavelengths.

These filters have been used to block spurious or unwanted light from the excitation and collection paths of an optical system. In the case of edge filters, filters having higher edge steepness (i.e., a smaller difference in wavelength over a defined transmission range) are capable of more effectively blocking spurious or unwanted light signals. Further, edge filters having lower transmission loss, if placed in the collection path, are capable of passing more light from the sample to the measuring instrument. Similarly, LLF's having lower transmission loss, if placed in the excitation path, are capable of passing more excitation light from the light source (e.g., a laser) to the sample.

Raman spectroscopy is one example of an optical analysis system that advantageously employs dichroic/interference filters. In Raman spectroscopy, molecular material is irradiated with excitation light, i.e., high intensity light of a given wavelength λ_(L) (or range of wavelengths) Upon irradiation, the molecular material scatters the excitation light. A small portion of the scattered excitation light is “Raman shifted,” i.e., it is shifted in wavelength above and/or below λ_(L). This Raman shifting is attributed to the interaction of the light with resonant molecular structures within the material, and the spectral distribution of the Raman shifted light provides a spectral “fingerprint” characteristic of the composition of the material. However, the bulk portion of the scattered excitation light is “Rayleigh scattered,” i.e., it is scattered without a shift in wavelength.

Because the amount of Raman shifted light is very small relative to the amount of Rayleigh scattered light, it is necessary to filter the Raleigh scattered light from the collection path before it reaches the detector. Without such filtering, the Rayleigh scattered light will swamp the detector, and may excite spurious Raman scattering in the collection path. Filtering of the Rayleigh scattered light can be accomplished, for example, by placing an edge filter, such as a LWP filter having a transition wavelength λ_(T) just above λ_(L) (or range of wavelengths) between the sample and the detector. In this position, the LWP filter ensures that the light reaching the detector is predominantly long-wavelength Raman-shifted light from the sample. Similar arrangements using edge filters can be used to analyze short wavelength Raman-shifted light.

In an ideal Raman spectroscopy setup, a filter, such as a notch or edge filter, is configured such that it blocks 100% of light having a wavelength λ_(L) (or range of wavelengths) from reaching the detector, while allowing desired light to be passed to the detector for measurement. This could be accomplished for example, if the filters were configured so as to exhibit an ideal stopband that blocks 100% of light having a wavelength λ_(L) (or range of wavelengths).

Conventional filters, however, exhibit narrow blocking or transmission bands that exhibit a level of transmission and/or blocking that is less than optimum. The “blocking” of a filter at a wavelength or over a region of wavelengths is typically measured in optical density (“OD” where OD=−log₁₀(T), T being transmission of the filter at a particular wavelength). Conventional filters that achieve high OD values at certain wavelengths or wavelength regions may not necessarily also achieve high transmission (in excess of 50%, for example) at any other wavelengths or wavelength regions. High OD is generally exhibited in a fundamental “stopband” wavelength region, and such stopbands have associated with them higher-order harmonic stopband regions occurring at other wavelength regions.

These higher-order stopbands are one reason why it is difficult to achieve high transmission at wavelengths shorter than those over which high blocking occurs. A stopband is a range of wavelengths over which transmitted light is strongly attenuated (T≦10%) due to constructive interference of the many partial waves of light reflected off of a structure with a periodic or nearly periodic variation of the index of refraction, as found in a thin-film interference filter. For a “quarter wavelength stack” structure comprised of alternating layers of high- and low-index materials, each of which is approximately one quarter of a particular wavelength λ₀ thick (in the material), the “fundamental” stopband is roughly centered on λ₀ and ranges from approximately λ₀/(1+x) to λ₀/(1−x), where x is related to the high and low index of refraction values, n_(H) and n_(L), respectively, according to

$x = {\frac{2}{\pi}{{\arcsin\left( \frac{n_{H} - n_{L}}{n_{H} + n_{L}} \right)}.}}$

If the layer-to-layer index of refraction variation is not a purely sinusoidal variation, but rather changes abruptly, as is typically the case in a multi-layer thin-film interference filter, higher-order stopbands exist at shorter wavelengths. For example, a quarter-wave stack having such abrupt refractive index changes exhibits “odd-harmonic” stopbands that occur approximately at the wavelengths λ₀/3, λ₀/5, etc., and that range from approximately λ₀/(3+x) to λ₀/(3−x), for the third-order stopband, λ₀/(5+x) to λ₀/(5−x), for the fifth-order stopband, and so on. If the layers are not exactly a quarter-wave thick, there may also be “even-harmonic” stopbands that occur approximately at the wavelengths λ₀/2, λ₀/4, etc.

In general, known filters achieve high blocking over a wide range by utilizing a fundamental stopband, by combining multiple fundamental stopbands, or by “chirping” (gradually varying) the layers associated with one or more fundamental stopbands. Regardless of the approach, the higher-order harmonic stopbands associated with these blocking layers inhibit transmission at wavelengths shorter than the fundamental stopband or stopbands.

FIG. 2 schematically illustrates a Raman spectroscopy system 10 having a standard configuration. As shown, this standard configuration includes a light source 1, such as a laser, an excitation filter 2, a sample 3, a collection filter 4, and a detector 5. In operation, light source 1 emits light having a wavelength λ_(L) (or range of wavelengths) which passes though excitation filter 2 and illuminates sample 3 directly. Sample 3 scatters Raman shifted and unshifted excitation (Rayleigh scattered) light. Collection filter 4 is positioned between sample 3 and detector 5, such as a spectrophotometer. Collection filter 4 is configured to block the Rayleigh scattered light from sample 3 but transmit as much of the Raman shifted light as possible, and as close to λ_(L) as possible.

In focusing or imaging systems that utilize high numerical aperture (high-NA) collection optics, however, it is desirable for light from the light source and the collected signal light to share a common path. To meet this requirement, a two-filter solution is ideal. FIG. 3 schematically illustrates a Raman spectroscopy system 20 having such a configuration.

As shown, this configuration generally includes a light source 11, such as a laser, an excitation filter 12, a sample 13, a collection filter 14, a detector 15, such as a spectrophotometer, and a beamsplitter optical filter 16. Beamsplitter optical filter 16 is oriented at non-zero angle of incidence, e.g., about 45°, relative to light incident from light source 11, and is configured to reflect incident light from light source 11 onto sample 13, while transmitting Raman scattered light returning from Sample 13. Collection filter 14 is used in conjunction with beamsplitter optical filter 16 to ensure complete blocking of incident light that is Rayleigh scattered or reflected from sample 13. Due to the orientation of beamsplitter optical filter 16 relative to light from light source 11, the system shown in FIG. 3 is configured for so called, “non-zero angle of incidence” spectroscopy.

Increasing the angle of incidence of a traditional interference filter from normal generally affects the spectrum of the filter in two ways. First, the features of the filter spectrum are shifted to shorter wavelengths. And second, as the angle of the filter is further increased from normal, the filter spectrum exhibits progressively increasing “polarization splitting.” That is, the filter produces two distinct spectra, one for s-polarized light, and one for p-polarized light. The relative difference between the s and p spectra at a given point is generally called “polarization splitting.”

To illustrate this principal, reference is made to FIGS. 4A and 4B which are plots of polarization splitting vs. angle of incidence for a quarter wave stack based on SiO₂ and Ta₂O₅ centered at 500 nm. In the plot of FIG. 4A, the bandwidths of the stopbands associated with light of s polarization and p polarization are shown, with the bandwidths measured in so-called “g-space.” The parameter g=λ₀/λ is inversely proportional to wavelength and therefore directly proportional to optical frequency, and equals 1 at the wavelength λ₀ which is at the center of a fundamental stopband associated with a stack of thin film layers each equal to λ₀/4 n in thickness, where n is the index of refraction of each layer. The bandwidth in g-space is therefore equal to the difference between λ₀/λ_(S) and λ₀/λ_(L), where λ_(S) and λ_(L) are the short-wavelength and long-wavelength edges of the stopband, respectively. The polarization splitting in g-space is thus simply one half of the difference between the bandwidths in g-space for s-polarized and p-polarized light. As shown in FIG. 4B, the stack exhibits polarization splitting of about 0.04 g-numbers when operated at 45° AOI. Increasing AOI to 60° results in polarization splitting of almost 0.08 g-numbers. Decreasing AOI to 20° results in polarization splitting of less than 0.02 g-numbers.

Many uses for thin film interference filters are known. For example, U.S. Pat. No. 7,068,430, which is incorporated herein by reference, discusses the use of such filters in Fluorescence spectroscopy and other quantification techniques.

Dichroic optical filters have been proposed for use in optical systems employing a two filter design, such as the one shown in FIG. 3. However, as described above and shown in FIG. 4, traditional dichroic filters exhibit substantial polarization splitting, particularly when operated at about 45° Angle of incidence. This polarization splitting arises from the particular construction of a dichroic filter. As mentioned previously, traditional dichroic filters are generally made up of alternating thin material layers having differing refractive index. In addition to the refractive index of each layer being different than that of an adjacent layer, the effective refractive indices of each individual layer differ with respect to different polarizations of light. That is, the effective refractive index for a layer is different for p-polarized light than it is for s-polarized light. As a result, s-polarized and p-polarized light are shifted to different degrees upon passing through each layer in a dichroic filter. This difference in shift ultimately offsets the filter spectra corresponding to these differing polarizations, resulting in polarization splitting.

If a traditional dichroic filter is based on the first order stopband of an angle-matched quarter-wave stack, estimating the polarization splitting between the stopband bandwidths of the filter is relatively straightforward. That is, assuming the dichroic filter is made up of two materials having indices of n_(H) and n_(L), respectively, at 45° angle of incidence, the effective indices can be calculated as follows:

$\begin{matrix} {n_{L}^{S} = \sqrt{n_{L}^{2} - {\sin^{2}({AOI})}}} & (1) \\ {n_{H}^{S} = \sqrt{n_{H}^{2} - {\sin^{2}({AOI})}}} & (2) \\ {n_{L}^{P} = \frac{n_{L}^{2}}{\sqrt{n_{L}^{2} - {\sin^{2}({AOI})}}}} & (3) \\ {n_{H}^{P} = \frac{n_{H}^{2}}{\sqrt{n_{H}^{2} - {\sin^{2}({AOI})}}}} & (4) \end{matrix}$ Wherein:

-   -   AOI is the incident angle in air, which is assumed to the         incident medium;     -   n_(L) ^(P) and n_(L) ^(S) are the effective refractive index of         the low index material in the dichroic stack for p-polarized         light and s-polarized light, respectively; \     -   n_(H) ^(P) and n_(H) ^(S) are the effective refractive index of         the high index material in the dichroic stack for p-polarized         light and s-polarized light, respectively; and     -   n_(H) ² and n_(H) ^(S) are the squares of the high and low         refractive indexes, respectively, associated with the two         materials, and which are independent of polarization.

The bandwidths and polarization splitting of the first-order stopband for the two polarizations may then be calculated as follows:

$\begin{matrix} {{\Delta\; g^{S}} = {\frac{4}{\pi}{\sin^{- 1}\left( \frac{n_{H}^{S} - n_{L}^{S}}{n_{H}^{S} + n_{L}^{S}} \right)}}} & (5) \\ {{\Delta\; g^{P}} = {\frac{4}{\pi}{\sin^{- 1}\left( \frac{n_{H}^{P} - n_{L}^{P}}{n_{H}^{P} + n_{L}^{P}} \right)}}} & (6) \\ {{PS}_{g} = \frac{{\Delta\; g^{S}} - {\Delta\; g^{P}}}{2}} & (7) \end{matrix}$ Wherein:

-   -   Δg^(S) and Δg^(P) are the bandwidths of the first order         (fundamental) stopband for s-polarized light and p-polarized         light, respectively, in g-space; and     -   PS_(g) is the polarization splitting for the first-order         stopband in g-space.         Alternatively, the polarization splitting may be expressed in         terms of wavelength. For example,

$\begin{matrix} {{PS}_{\lambda} = {\frac{\lambda_{0}}{1 - {\Delta\;{g^{S}/2}}} - \frac{\lambda_{0}}{1 - {\Delta\;{g^{P}/2}}}}} & (8) \end{matrix}$ wherein:

-   -   PS_(λ) is the polarization splitting of the long-wavelength edge         of the fundamental stopband (the edge associated with a         long-pass filter).         Often this value is expressed as a dimensionless value by taking         its ratio to the average wavelength of the edges associated with         s- and p-polarizations and expressing it as a percentage.

Polarization splitting has been utilized to design polarizing filters where high transmission and blocking are achieved for s and p polarizations, respectively, over a defined wavelength band. However, in the context of edge filters and beamsplitter optical filters, polarization splitting severely limits the edge steepness of light having average polarization. Thus, it is desirable to minimize polarization splitting as much as possible.

Several ways have been proposed to minimize polarization splitting. For example, one method proposed by Thelen (See A. Thelen, “Design of Optical Interference Coatings,” McGraw Hill, 1989) utilizes tuning spacers of a multi-cavity Fabry-Perot bandpass filter to align the edges of spectrum of s and p-polarized light. However, this method has significant limitations when used to create dichroic filters.

In Thelen's method, the starting layer structure is that of a multi-cavity Fabry-Perot bandpass filter with spacer layers having optical thickness equal to multiple half-waves of the reference wavelength used to define the associated stopband. In addition, the edge of the resulting dichroic must be essentially at the center of the associated stopband. This is unlike the filters according to the present disclosure discussed below, which differ from Thelen's approach both in layer structure and placement of the dichroic edge with respect to the stopband. Indeed, as discussed below, filters according to the present disclosure do not contain the spacer layers required by Thelen's approach, and the dichroic edge may be placed virtually anywhere with respect to the location of the stopband.

In addition, it has been shown that decreasing stopband bandwidth can result in a corresponding decrease in polarization splitting. In the case of a filter having a second order stopband, the bandwidth of the stopband is proportional to the material mismatch in the dielectric stack making up the filter, where “mismatch” refers to the deviation of the layer thicknesses from one quarter of a wavelength, while keeping the sum of the thicknesses of each pair of high- and low-index layers equal to approximately one half of a wavelength. The greater the mismatch, the higher the degree of polarization splitting, and vice versa. Thus, it has been shown that polarization splitting can be minimized by utilizing different (e.g., higher-order) stopbands and adjusting material mismatch in the dielectric stack making up a dichroic filter.

However, while this method is effective, small mismatch always results in a filter having a narrow blocking region and lower blocking level, which is often not acceptable. Enhancement of the blocking region can be achieved, but only by increasing the number of layers in the dielectric stack. As a result, the performance of a traditional dichroic filter based on a second order stopband is typically limited by the maximum coating thickness allowed by the manufacturing process.

In addition, dual notch dichroic beamsplitters have been proposed for use in optical systems having dual filter designs. FIG. 5 is a measured spectrum of unpolarized light passing through an exemplary dual notch dichroic beamsplitter. As shown, this filter exhibits two narrow stopband regions 62 and 64 separated by a passband region having very narrow bandwidth 66. The spectrum also exhibits a relatively narrow bandpass region 68 between stopband region 64 and a fundamental stopband above about 750 nm (not shown)

While prior known interference filters are useful for many applications, they generally exhibit unsatisfactory characteristics when operated at about 45° angle of incidence. For example, the dual notch filter shown in FIG. 5 exhibits polarization splitting of 0.58% at one edge of stopband 62, and 0.4% at one edge of stopband 64. However, this filter exhibits a relative passband bandwidth of only about 30%, which is unsatisfactory. The relative passband bandwidth is the ratio of the difference between the long-wavelength of the passband and the dichroic edge wavelength to the dichroic edge wavelength (for a long-pass dichroic filter). Further, this filter exhibits relatively poor edge steepness of 1.26% at one edge of stopband 62, and 0.79% at one edge of stopband 64, which are insufficient for many applications. The edge steepness here is defined as the normalized wavelength difference between 10% and 90% transmission wavelengths for average polarized light.

Finally, angle matched notch filters have also been proposed for use in non-zero angle of incidence spectroscopy. Notch filters are described in detail in U.S. Pat. No. 7,123,416, the contents of which are incorporated herein by reference. However, when these filters are operated at about 45° Angle of incidence, they suffer from significant polarization splitting, as shown in FIG. 6 (where 72, 74, and 76 correspond to the s spectrum, p spectrum, and average spectrum, respectively) F and described above. Accordingly, these types of filters exhibit significant limitations when used in many optical measurement techniques.

Thus, there is a need for improved interference filters that, when operated at about 45° angle of incidence, exhibit substantially improved properties relative to prior known filters. In particular, there is a desire in the art for improved interference filters that, when operated at about 45° angle of incidence, exhibit at least one of improved polarization splitting, passband bandwidth, edge steepness, and blocking, relative to prior known filters.

SUMMARY OF THE DISCLOSURE

The present disclosure provides optical interference filters that are suitable, for example, for use in Raman spectroscopy, fluorescence imaging, and/or quantification applications. Among other things, these filters exhibit substantially better performance characteristics when operated at about 45° angle of incidence, relative to prior known interference filters. In particular, the present disclosure provides optical filters that exhibit at least one of improved polarization splitting, edge steepness, passband bandwidth, and blocking, relative to prior known interference filters.

Consistent with the present disclosure are optical filters that include a substrate and a plurality of alternating first and second material layers on the substrate. The alternating first and second material layers have respectively different refractive indices. For the purposes of this disclosure, this structure is referred to as the “basic structure.”

As will be discussed at length below, the plurality of alternating material layers of filters in accordance with the present disclosure may be configured so as to achieve one or more of a variety of desired optical characteristics. In some embodiments, these layers are configured so as to obtain filters that exhibit, when light impinges on the filter at about 45° angle of incidence, at least one spectrum having a first stopband region and a second stopband region separated by a passband region. Thus, filters in accordance with the present disclosure may, for example, exhibit a first spectrum for p-polarized light, a second spectrum for s-polarized light, and an average spectrum corresponding to the average of the s and p spectra.

In addition, the plurality of first and second material layers can be configured such that the first stopband region correlates to a fundamental stopband of the filter, whereas the second stopband correlates to a harmonic of the first stopband region, or a non-harmonic of the first stopband region, such as a passband defect. A non-harmonic stopband is a stopband that occurs in one of the passband regions on either side of a fundamental stopband, and does not occur at a wavelength which is an odd or even harmonic of the fundamental stopband. A non-harmonic stopband may be created by optimizing the thicknesses of the nearly quarter-wavelength-thick layers which form the fundamental stopband in such a way as to cause the optical interference of light in the layer structure to exhibit strong reflection over a region within one passband, while exhibiting high transmission with relatively low ripple over the remaining portion of the passband. Hence, when formed this way, this type of stopband is referred to here as a “passband defect.”

Further, the plurality of first and second material layers in filters in accordance with the present disclosure may be configured so as to optimize one or more characteristics of the filter spectrum. For example, the plurality of layers may be configured so as to optimize at least one of polarization splitting, edge steepness, blocking, and passband bandwidth exhibited by the filter spectrum, particularly when the filter is operated at about 45° angle of incidence. In some embodiments, filters in accordance with the disclosure may be configured so as to optimize two or more of these features relative to one another. Moreover, these filters may be configured so as to optimize at least one of the aforementioned characteristics in at least one region of the filter spectrum, such as at the edge or base of a stopband region.

The present disclosure also describes methods of making the optical filters described herein, as well as systems using the optical filters described herein. Thus, consistent with the present disclosure are optical filters having the structure described herein, and which are produced by known deposition techniques, such computer controlled ion beam sputtering.

Also consistent with the present disclosure are optical systems that incorporate at least one of the filters described herein as an optical filter. For example, these systems may include the filters described herein as an edge, laser line, or dichroic beamsplitter filter for non-zero angle of incidence spectroscopy. Of course, the filters described herein may also be used in other systems and in other ways consistent with the use of previously known optical filters.

Additional objects and advantages of the disclosure will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the disclosure. The objects and advantages of the disclosure will be realized and attained by means of the elements and combinations particularly pointed out in the appended claims.

It is to be understood that both the foregoing general description and the following detailed description are exemplary and explanatory only and are not restrictive of the disclosure, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

The advantages, nature, and various additional features of the disclosure will appear more fully upon consideration of the illustrative embodiments described below in detail in connection with the accompanying drawings. In the drawings:

FIG. 1A is a spectrum of an idealized long wave pass interference filter.

FIG. 1B is a spectrum of an idealized short wave pass interference filter.

FIG. 2 is a schematic illustration of a Raman spectroscopy system having a standard configuration

FIG. 3. is a schematic illustration of a Raman Spectroscopy System having a two-filter configuration.

FIG. 4A is a plot of bandwidth in g-space vs. angle of incidence for a traditional quarter wave stack based on SiO₂ and Ta₂O₅ centered at 500 nm.

FIG. 4B is a plot of polarization splitting vs. angle of incidence for a traditional quarter wave stack based on SiO₂ and Ta₂O₅ centered at 500 nm.

FIG. 5 is a spectrum of light of average polarization passing through a dual notch dichroic beamsplitter.

FIG. 6 is a design spectrum of an angle matched notch filter measured at 45° angle of incidence

FIG. 7 is a calculated spectrum of a minimal polarization splitting dichroic filter in accordance with the present disclosure.

FIG. 8 is a calculated spectrum of a minimal polarization splitting dichroic filter in accordance with the present disclosure.

FIG. 9 is a comparison of a dichroic design in accordance with the present disclosure and a traditional angle matched short wave pass filter having comparable coating thickness.

FIG. 10 is a design spectrum of a 532 nm steep dichroic beamsplitter in accordance with the present disclosure.

FIG. 11 is a magnified portion of a design spectrum of a 532 nm steep dichroic beamsplitter in accordance with the present disclosure.

FIG. 12 is a magnified portion of a measured spectrum of a 532 nm steep dichroic beamsplitter in accordance with the present disclosure.

FIG. 13 is a simulation of a measured spectrum of a 532 nm steep dichroic beamsplitter, using a 2-degree cone-half angle beam at the filter.

FIG. 14 is a design spectrum of a 785 nm steep dichroic beam splitter in accordance with the present disclosure.

FIG. 15 is a magnified portion of a design spectrum of a 785 nm steep dichroic beam splitter in accordance with the present disclosure.

FIG. 16 is a measured spectrum of a deeply blocking, steep 532 nm edge filter measured at 45° Angle of incidence.

FIG. 17. is a magnified portion of a design spectrum of a deeply blocking, steep 532 nm edge filter measured at 45° Angle of incidence.

DETAILED DESCRIPTION

Reference will now be made in detail to various exemplary embodiments of the present disclosure, examples of which are illustrated in the accompanying drawings. Wherever possible, the same reference numbers will be used throughout the drawings to refer to the same or like parts.

One aspect of the present disclosure relates to optical interference filters that exhibit improved characteristics when operated at about 45° Angle of incidence, relative to traditional optical interference filters.

As used herein, the term, “about 45° angle of incidence” means that the filter in question is oriented such that light from a light source impinges on a surface of the filter at an angle ranging from about 40° to about 50°, unless otherwise specified. Of course, the filters according to the present disclosure may be operated at any other angle of incidence. For example, the filters according to the present disclosure may be operated at an angle of incidence chosen from about 43° to about 48°, from about 44° to about 46°, and substantially 45°. In some embodiments of the present disclosure, the filters described herein are operated at substantially 45° angle of incidence.

All of the optical filters according to the present disclosure generally include the same basic structure. That is, they generally include a transparent substrate and a plurality of alternating first and second material layers deposited on a surface of the substrate. The plurality of alternating first and second material layers have respectively different refractive indices.

A wide variety of materials may be used to form the alternating first and second material layers. Among such materials, non-limiting mention is made of metals, metallic and non-metallic oxides, transparent polymeric materials, and so called “soft” coatings, such as sodium aluminum fluoride (Na₃AlF₆) and zinc sulfide (ZnS). Further non-limiting mention is made of metallic oxides chosen from SiO₂, Ta₂O₅, Nb₂O₅, HfO₂, TiO₂, and Al₂O₅. In some embodiments of the present disclosure, the first and second material layers are Nb₂O₅ and SiO₂, respectively

Filters in accordance with the present disclosure may be manufactured using deposition methods and techniques that are known in the art. For example, these filters may be made with a computer controlled ion beam sputtering system, such as the one described in U.S. Pat. No. 7,068,430, which is incorporated herein by reference. In general, such a system is capable of depositing a plurality of alternating material layers, wherein the thickness of each layer may be precisely controlled.

Further, filter designs in accordance with the present disclosure may be produced by known thin-film filter design techniques. For example, these filter designs may be produced by optimizing the filter spectra and structure of an initial design, such as a traditional short wave pass or long wave pass interference filter against a target spectrum using known optical optimization routines. Non-limiting examples of such optimization routines include the variable-metric or simplex methods implemented in standard commercial thin-film design software packages, such as TFCalc by Software Spectra, Inc. and The Essential Macleod by Thin Film Center, Inc. A detailed description of filter design techniques that can be used to produce filter designs according to the present disclosure may be found in U.S. Pat. No. 7,068,430, which is incorporated herein by reference.

The filters of the present disclosure differ from traditional interference filters in that during production, the individual thicknesses of the alternating material layers making up the interference stack are carefully controlled so as to achieve desired optical characteristics that are not exhibited by prior known optical filters. For example, optical filters consistent with the present disclosure may be configured so as to exhibit, when operated at about 45° angle of incidence, at least one of improved polarization splitting, edge steepness, passband bandwidth, and blocking, relative to prior known interference filters.

Accordingly, one aspect of the present disclosure relates to interference filters having the basic structure described above, wherein the alternating first and second material layers are configured such that when light from a light source impinges on the filter at an angle of incidence of about 45°, the filter defines a spectrum for s-polarized light, a spectrum for p-polarized light, and an average spectrum corresponding to light having an average polarization (i.e., corresponding to an average of the s and p spectra). Each of these spectra includes a first stopband region and a second stopband region separated by a passband region.

As a non-limiting example of the basic spectral characteristics of filters according to the present disclosure, reference is made to FIGS. 7 and 8, which plot transmittance vs. wavelength for exemplary dichroic filters that are in accordance with the present disclosure, and which exhibit minimal polarization splitting. As shown in both of these FIGS., these exemplary filters exhibit a spectrum for s-polarized light, a spectrum for p-polarized light, and an average spectrum corresponding to an average of the s and p spectra. Each of these spectra exhibit a passband region 82, 92, a first stopband region 84, 94, and a second stopband region 86, 96. Of course, the location and width of the stopband regions and the passband of the filters according to the present disclosure are not limited to those shown in these FIGS.

As used herein, the term “stopband region,” means a range of wavelengths over which transmitted light is strongly attenuated (i.e., transmission is ≦10%) due to interference of the many partial waves of light reflected off of a structure with a periodic or nearly periodic variation of the index of refraction, as found in a thin-film interference filter. In the case of the filters described herein, light that is not transmitted is generally reflected, though blocking by other means (e.g., absorption) is also possible.

The first stopband region 84, 94 and second stopband regions 86, 96 may be centered on any wavelength region of the electromagnetic spectrum, so long as they do not overlap with one another. For example, the first and second stopband regions may encompass distinct wavelength ranges in the 200-1250 nm portion of the electromagnetic spectrum. In some embodiments of the present disclosure, the first and second stopbands are present in the 350-1250 nm range, such as the 350-850 nm range, more specifically the 400-750 nm range.

The second stopband region, such as regions 86, 96, may encompass wavelengths that are shorter or longer than those encompassed by the first stopband region. In some embodiments, the second stopband is placed so as to attenuate and/or block substantially monochromatic light of a given wavelength. For example, the second stopband may be placed so as to block laser light having a wavelength within the 200-1250 nm range of the electromagnetic spectrum. In some non-limiting embodiments, the second stopband is placed so as to attenuate and/or block substantially monochromatic light having a wavelength chosen from 488.0 nm, 514.5 nm, 532.0 nm, 623.8 nm, and 785.0 nm.

Placement of the cut-on edge and/or cut-off edge of the first and/or second stopband regions may be controlled by optimizing the layer thickness of the individual first and second material layers. Thus, for example, filters in accordance with the disclosure may be configured such that an edge wavelength (λ_(EW)) of at least one of the first and/or second passband region is located at wavelengths that are 2% or less from incident monochromatic light having a wavelength λ_(L). That is, |λ_(EW)−λ_(L)|/λ_(L)*100% may be 2% or less. Of course, λ_(EW) may be located closer to or farther from λ_(L), such as ≦1%, and ≦0.5%.

In some embodiments of the present disclosure, the plurality of alternating material layers are configured such that the first stopband region corresponds to a fundamental stopband of the filter, and the second stopband region corresponds to a an odd or even harmonic stopband of the fundamental stopband.

In other non-limiting embodiments, filters in accordance with the present disclosure may be configured such that the first stopband region corresponds to a fundamental stopband of the filter, and the second stopband region corresponds to a non-harmonic stopband region, such as a “passband defect.” That is, in these embodiments, the second stopband region encompasses a range of wavelengths that do not correspond to an odd or even harmonic of the fundamental (first) stopband region. For a more specific description of fundamental and harmonic stopbands, reference is made to U.S. Pre-Grant Publication No. 2008-0037129, the contents of which are incorporated herein by reference.

Filters with a passband defect may be created in a variety of ways. As an example, a long-pass dichroic filter based on a passband defect to the short-wavelength side of a fundamental stopband may be obtained from a starting structure correlating to a quarter-wave stack consisting of plurality of alternating first and second material layers having different refractive indexes. The quarter-wave optical thickness is defined with respect to a reference wavelength chosen such that the associated fundamental stopband is above the desired transmitting wavelength region of the target LWP dichroic. The location of the passband defect generally does not coincide with the higher harmonic stopbands associated with the fundamental stopband, and therefore occurs in a region that is transmitting prior to any optimization of layer thickness. Once the starting structure is established, thin-film filter optimization algorithms known in the art may be used to gradually increase the blocking level over the passband defect wavelength region, and then to optimize the layer structure after each increase, until the target blocking level is achieved.

Regardless of whether the second stopband region correlates to a harmonic or non-harmonic stopband of the first stopband region, it is possible through careful control of the configuration of the plurality of alternating first and second material layers to optimize various aspects of the filter spectrum, as described below.

In some non-limiting embodiments, filters according to the present disclosure are configured so as to maximize edge steepness in a region of the filter spectrum corresponding to at least one edge wavelength of the first and/or second stopband regions, particularly when the filter is operated at about 45° Angle of incidence. As used herein, and unless otherwise specifically stated, the term “edge steepness,” refers to the relative difference (in percent) of the wavelength of a spectrum of light having average polarization at the 10% transmission point (λ₁₀) and the 90% transmission point (λ₉₀) of a long or short wave edge of the first and/or second passband region, relative to a corresponding edge wavelength of the relevant stopband. That is, “edge steepness” (ES) is defined by the expression: ES=(|λ₉₀−λ₁₀|/λ_(EW))*100% wherein λ_(EW) is the corresponding edge wavelength.

Further, as used herein, the term, “corresponding edge wavelength,” refers to the cut-on or cut-off frequency of the first or second stopband region for light of average polarization corresponding to the particular edge under consideration. Thus, for example, if the passband region of the filter spectrum encompasses wavelengths longer than the second stopband region such as shown in FIG. 7, λ_(EW) refers to the cut-on frequency 87 of the second stopband (the long wave edge of the second stopband), or the cut-off frequency 88 of the first stopband (the short wave edge of the first stopband). The opposite is true if the passband region encompasses wavelengths shorter than said second stopband region. That is, in those cases, “edge wavelength” (λ_(EW)) refers to the cut-off frequency of the second stopband (i.e., the short wave edge of the second stopband) and the cut-on frequency of the first stopband (the long wave edge of the first stopband), for light of average polarization.

According to some embodiments, filters consistent with the present disclosure have the basic structure described above, and define at least one spectrum having the general features described above when the filter is operated at about 45° Angle of incidence. Moreover, these filters may be configured such that at least one edge of the first and second stopband regions exhibits an edge steepness ranging from ≦0.76%, ≦0.75%, ≦0.65%, ≦0.50%, ≦0.40%≦0.25%, ≦0.23%, ≦0.17%, and ≦0.10% or less.

In non-limiting embodiments, filters consistent with the present disclosure are configured such that at least one edge of the second stopband region has an edge steepness within the above described ranges, wherein the second stopband region correlates to a harmonic or non-harmonic of the first stopband region. For example, the filters may be configured such that the second stopband region of the filter spectrum exhibits an edge steepness within these ranges, and correlates to a passband defect.

Also in accordance with the present disclosure are interference filters having the basic structure described above, and which exhibit, when the filter is operated at about 45° angle of incidence, at least one filter spectrum having the general features described above. In these non-limiting embodiments, the filters are configured such that at least one of the first and second stopband regions exhibit a defined edge steepness between the optical density 5 and 90% transmission points and/or the optical density 3 and 90% transmission points of the filter spectrum for light of average polarization. For example, filters in accordance with the present disclosure may be configured so as to exhibit an edge steepness, between the optical density 5 and 90% transmission points of least one of the first and second stopband regions, that ranges from about ≦1.82%, ≦1.26%, ≦1.0%, ≦0.75%, ≦0.50%, and ≦0.46% or less, relative to a corresponding edge wavelength. Similarly these filters may be configured such that they exhibit an edge steepness between the optical density 3 and 90% transmission points ranging from about ≦0.76%, ≦0.75%, ≦0.56%, ≦0.50%, ≦0.25%, and ≦0.24% or less, relative to a corresponding edge wavelength.

In non-limiting embodiments, at least one edge of the second stopband region exhibits an edge steepness within these ranges, wherein the second stopband correlates to a harmonic or non-harmonic of the first stopband region.

In still other non-limiting embodiments, filters according to the present disclosure may be configured such that the first and second stopband regions each comprise a long wave edge and a short wave edge. For example, as shown in FIG. 9, first stopband 102 and second stopband 103 exhibit short wave edges 107 and 108 respectively, and long wave edges 109 and 110, respectively. Short wave edges 107, 108 have a wavelength (or range of wavelengths) λ_(S1), and λ_(S2), respectively, and long wave edges 109, 110, have a wavelength (or range of wavelengths) λ_(L1) and λ_(L2). At least one of long wave edges 109, 110 and short wave edges 107, 108 have an edge steepness, relative to a corresponding edge wavelength, falling within the above described ranges. In a non-limiting embodiment, at least one of short wave edge 107 and long wave edge 110 have an edge steepness falling within the above described ranges.

The plurality of alternating material layers in the filters according to the present disclosure may also be configured so as to optimize polarization splitting exhibited by the filter spectrum.

Thus, consistent with the present disclosure are interference filters having the same basic structure and general spectral characteristics described above. The plurality of alternating material layers of the filter are configured so as to minimize polarization splitting when the filter is operated at about 45° Angle of incidence.

As used herein, the term, “polarization splitting” refers to the difference (in percent) between a wavelength for s-polarized light (λ_(50S)) and a wavelength for p-polarized light (λ_(50P)), relative to an average of λ_(50S) and λ_(50P), wherein λ_(50S) and λ_(50P) are measured at a 50% transmission point of an edge of a stopband region of the corresponding s and p spectra. That is, polarization splitting (PS) at a given edge of a stopband is defined by the relation: PS=(|λ_(50S)−λ_(50P))/[λ_(50S)+λ_(50P))/2]

In some embodiments, the filters according to the present disclosure are configured so as to exhibit, when operated at 45° angle of incidence, at least one spectrum including a first stopband region and a second stopband region separated by a passband region, wherein the first or second stopband region of the filter exhibits polarization splitting chosen from about ≦0.50%, ≦0.25%, ≦0.10%, ≦0.039%, ≦0.033%, and ≦0.015% or less. For example, filters according to the present disclosure may be configured such that the first stopband region correlates to a fundamental stopband, the second stopband region correlates to a harmonic of the first stopband, and the second stopband region exhibits polarization splitting within the above described ranges when the filter is operated at about 45° angle of incidence.

In other non-limiting embodiments, filters according to the present disclosure may be configured such that the second stopband region correlates to a non-harmonic stopband (such as a passband defect) of the first stopband region, and the second stopband region exhibits polarization splitting within the above described ranges when the filter is operated at about 45° angle of incidence.

Also consistent with the present disclosure are interference filters that have the same basic structure described above, and which exhibit both low polarization splitting and wide passband bandwidth relative to a corresponding edge wavelength (λ_(EW)).

As used herein, the term, “passband bandwidth,” means the width (in percent) of the passband region separating the first and second stopband regions of the filter spectrum, relative to the corresponding edge wavelength λ_(EW) of the second stopband region. Thus, for example, if a passband is present between a longwave edge (λ_(LW2)) of the second stopband region, and a shortwave edge (λ_(SW1)) of the first stopband region, the passband bandwidth (PB) is defined by the expression: PB=(|λ_(SW1)−λ_(LW2)|/λ_(EW))*100%, where λ_(EW) is defined as indicated above for the relevant edge of the second stopband region.

Thus, in some embodiments, filters in accordance with the present disclosure may exhibit, for example, ≦0.5% polarization splitting, such as ≦0.25% polarization splitting, in conjunction with a defined passband bandwidth. For example, filters in accordance with present disclosure may be configured such they exhibit, when operated at about 45° Angle of incidence, polarization splitting within the above described ranges in conjunction with a passband bandwidth ranging from about ≧58.85%, ≧55.97%, ≧50.00%, ≧30.00%, ≧25.00%, ≧23.52%, ≧10.00%, ≧8.06%, ≧7.66%, and ≧5.00%. For example, filters according to the present disclosure may be configured such that the second stopband region exhibits polarization splitting within the above described ranges, and correlates to a harmonic of the first stopband region or a non-harmonic of the first stopband region, such as a passband defect.

The achievable passband bandwidth in filters according to the present disclosure is dependent upon the edge steepness and/or polarization splitting. The steeper the edge or the smaller the polarization splitting, the more difficult it is to achieve a wider passband.

Also consistent with the present disclosure are interference filters having the same general structure described above, and which exhibit extended and/or enhanced blocking in a wavelength range corresponding to at least one of the first and second stopband regions when operated at about 45° Angle of incidence.

Extended blocking about the first and/or second stopband regions may be accomplished by adding additional layer structure to the plurality of alternating material layers present in filters according to the present disclosure. The addition of extended blocking to a complex filter coating is described in detail in U.S. Pat. No. 6,809,859, which is incorporated herein by reference. Similarly, blocking within the first and or second stopband regions of filters according to the present disclosure may be enhanced by depositing additional alternating first and second material layers.

In this way, filters according to the present disclosure may be configured to provide deep blocking of wavelengths within at least one of the first and second stopband regions. That is, filters according to the present disclosure may be configured so as to transmit about 10%, 5%, 1%, or substantially 0% (i.e., optical density 6) of wavelengths falling within at least one of the first and second stopband regions. In non-limiting embodiments, filters according to the present disclosure are configured such that the second stopband region exhibits blocking within the above described ranges, and correlates to a harmonic of the first stopband region or a non-harmonic of the first stopband region, such as a passband defect.

Filters according to the present disclosure can improve the performance of a variety of optical analysis systems that illuminate/excite a sample with light of a first wavelength (or range of wavelengths) to produce a measurable or viewable response of light at a second wavelength different from the first. Such systems, which include Raman spectroscopy and fluorescence microscopy, typically have the typical construction shown in FIG. 2, or the two-filter construction shown in FIG. 3.

Filters according to the present disclosure may be used in known optical systems in any manner consistent with the use of interference filters known in the art. For example, filters according to the present disclosure may be used in optical systems employing the two filter configuration shown in FIG. 3. As previously described, such a system generally includes a light source 11, such as a laser, an excitation filter 12, a sample 13, a collection filter 14, a detector 15, and a beamsplitter optical filter 16. Beamsplitter optical filter 16 is oriented at non-zero angle of incidence, e.g., about 45°, relative to light incident from light source 11, and is configured to reflect incident light from light source 11 onto sample 13, while transmitting scattered light having a corresponding shift in wavelength (e.g., Raman scattering) returning from Sample 13. Collection filter 14 is used in conjunction with beamsplitter optical filter 16 to ensure complete blocking of incident light that is Rayleigh scattered or reflected from sample 13. Alternatively, beamsplitter optical filter 16 may itself have high blocking at the excitation wavelength, thus obviating the need for the then redundant collection optical filter 14.

Filters in accordance with the present disclosure may be used, for example, as beamsplitter optical filter 6 in optical systems of the general two filter configuration shown in FIG. 3. In this case, the alternating first and second material layers may be configured such that when light from the light source impinges on the dichroic beamsplitter optical filter at an angle of incidence of about 45°, the filter defines a spectrum for s-polarized light and a spectrum for p-polarized light, with each spectra defining a first stopband region and a second stopband region separated by a passband region. The layers are also configured such that the dichroic beamsplitter optical filter exhibits at least one of improved polarization splitting, edge steepness, blocking, and passband bandwidth, as described above.

Use of filters according to the present disclosure in such systems allows signals to be measured closer to the wavelength or wavelength region associated with the excitation laser or source, while maintaining necessary high blocking of the source light from the detection system. Thus, in Raman spectroscopy, filters according to the present disclosure allow the measurement of signals closer to the laser line. As a result, vibrational lines with very small energy shifts can be measured, thus providing information about a measured sample that would otherwise be obscured by Rayleigh scattered light. In fluorescence spectroscopy and imaging, the ability to measure signals closer to the source wavelength means that more signal can be captured, thus increasing the sensitivity of the system (ability to measure very small signals) and the specificity of the system (decrease in background noise). Furthermore, filters according to the present disclosure that exhibit enhanced blocking may allow for one or more of the excitation and/or collection filters of the system shown in FIG. 3 to be removed.

The disclosure will be more fully illustrated using the following non-limiting examples.

Other than in the examples, or where otherwise indicated, all numbers expressing endpoints of ranges, and so forth used in the specification and claims are to be understood as being modified in all instances by the term “about.” Accordingly, unless indicated to the contrary, the numerical parameters set forth in the specification and attached claims are approximations that may vary depending upon the desired properties sought to be obtained by the present disclosure. At the very least, and not as an attempt to limit the application of the doctrine of equivalents to the scope of the claims, each numerical parameter should be construed in light of the number of significant digits and ordinary rounding approaches.

Notwithstanding that the numerical ranges and parameters setting forth the broad scope of the present disclosure are approximations, unless otherwise indicated the numerical values set forth in the specific examples are reported as precisely as possible. Any numerical value, however, inherently contains certain errors necessarily resulting from the standard deviation found in their respective testing measurements.

EXAMPLES Examples 1 and 2 Minimal Polarization Splitting, Long Wave Pass Steep Edge Dichroic Filter Configurations Based on the Concept of Passband Defect

A filter design corresponding to minimal polarization long wave pass steep edge dichroic filter based on the concept of passband defect was produced by optimizing a traditional dichroic short wave pass (SWP) filter spectrum and structure against a design spectrum using well-known optimization algorithms (e.g., the variable metric approach). That is, this design was optimized starting from a dichroic SWP filter comprising a substrate and approximately 150 alternating quarter wavelength thick layers of materials having high and low refractive index at a reference wavelength, respectively, and in view of a target (design) spectrum having desired spectral characteristics.

In the design spectrum, the edge of the SWP was chosen to be slightly longer than a specified long wavelength edge of the dichroic passband. The passband ripple of the design spectrum was optimized. After optimizing the passband ripple, the optimization continued while the blocking level just below the cut-on wavelength of the passband defect was gradually increased.

The spectra of this filter design was calculated at 45° angle of incidence. These calculated spectra are shown in FIG. 7. As shown, this filter design exhibits average spectrum 83, s-spectrum 84, p-spectrum 85, a first stopband region 84 above about 560 nm, a second non-harmonic stopband region 86 ranging from about 450 to 508 nm, and a passband region 82 between the first and second stopband regions 84, 86. Moreover, the second stopband 86 region exhibits minimal polarization splitting (0.039%, or less than 0.2 nm) at its long wave edge 87.

The calculated s and p spectra of example filter 1 were compared to the spectra of a comparative filter (comparative example 1) that exhibited only a fundamental stopband, and no passband defect. This comparison is shown in FIG. 9. In this FIG., spectra 105 and 205 correlate to the spectra for s-polarized and p-polarized light, respectively, of example filter 1 (identified as “Filter 2” in the figure legend). Spectra 101 and 201 correlate to the spectra for s-polarized and p-polarized light, respectively, of the comparative filter (identified as “Filter 1” in the figure legend). As shown, example filter 1 exhibited spectral characteristics similar to those of the comparative filter, except that the filter of example 1 exhibited a passband defect to the short-wavelength side of the fundamental stopband.

The calculated edge steepness and polarization splitting of the filter of example 1 was also compared to those of a commercially available standard dichroic filter known in the art, i.e., the filter manufactured by Semrock, Inc. under part number FF506-Di02. The resulting data is reproduced in the table below.

TABLE 1 Comparison of a filter design in accordance with the present disclosure and a conventional dichroic filter design: Coating Average Polarization Edge S-Pol P-Pol Polarization Thickness 2% 10% 90% Steepness 50% 50% Splitting Comparative 7.7 504.4 507.2 510.9 3.7 510.5 508.6 1.9 example 1 example 1 12.1 506.9 507.7 508.8 1.1 508.4 508.3 0.2 * Edge Steepness is defined as the transition width between 10% and 90% transmission levels. As shown, the filter of example 1 exhibited significantly better edge steepness and polarization splitting, relative to comparative example 1.

A second filter design (example 2) similar to example filter 1 above was also produced. The calculated average spectrum for this design (FIG. 8) was compared to the spectrum of the standard angle-matched notch filter shown in FIG. 6. While the calculated spectra for the filter of example 2 exhibited relatively limited bandwidth compared to that of the notch filter, as shown in the following table, it achieves better edge steepness for unpolarized light, and comparable edge steepness for s-polarized light with a coating thickness less than half that of the notch filter.

TABLE 2 comparison of a second filter design in accordance with the present disclosure and a conventional notch filter: Coating Average Polarization S Polarization Thickness (μ) OD 5 90% ES OD 5 90% ES Example 2 13.4 502.5 508.8 6.3 505.0 508.8 3.8 Comparative 31.5 534.4 541.9 7.5 538.9 542.0 3.1 example 2 (notch filter) * Edge Steepness is defined as the transition width between OD 5 and 90% transmission level

Examples 3 and 4 532 nm Steep Dichroic Beamsplitter

A first 532 nm dichroic beamsplitter optical filter (example 3) was produced having the configuration shown in the following table:

TABLE 3 Design structure of a 532 nm steep dichroic beamsplitter 532 nm Steep Dichroic Beamsplitter Coating Thickness (μm): 14.572006282 Total Layers: 92 Layer Material Thickness (nm) 1 Nb2O5 15.76397 2 SiO2 299.964436 3 Nb2O5 140.250246 4 SiO2 191.877075 5 Nb2O5 127.915529 6 SiO2 182.984338 7 Nb2O5 127.387605 8 SiO2 179.120336 9 Nb2O5 127.717542 10 SiO2 175.632888 11 Nb2O5 128.592224 12 SiO2 176.380586 13 Nb2O5 128.373916 14 SiO2 178.887283 15 Nb2O5 127.769399 16 SiO2 184.194878 17 Nb2O5 127.93426 18 SiO2 188.227308 19 Nb2O5 128.005756 20 SiO2 188.057134 21 Nb2O5 127.918216 22 SiO2 184.475321 23 Nb2O5 128.072983 24 SiO2 179.425546 25 Nb2O5 128.141557 26 SiO2 176.964355 27 Nb2O5 127.96974 28 SiO2 177.506078 29 Nb2O5 126.511604 30 SiO2 178.33606 31 Nb2O5 122.577018 32 SiO2 184.039011 33 Nb2O5 117.335837 34 SiO2 194.189076 35 Nb2O5 112.228887 36 SiO2 203.60499 37 Nb2O5 106.362721 38 SiO2 211.476307 39 Nb2O5 104.05433 40 SiO2 216.082874 41 Nb2O5 104.97781 42 SiO2 216.022612 43 Nb2O5 105.579945 44 SiO2 215.384734 45 Nb2O5 109.206235 46 SiO2 216.710917 47 Nb2O5 112.573883 48 SiO2 215.791662 49 Nb2O5 111.467447 50 SiO2 214.359317 51 Nb2O5 112.161114 52 SiO2 215.045267 53 Nb2O5 111.2683 54 SiO2 213.279207 55 Nb2O5 110.267674 56 SiO2 214.458176 57 Nb2O5 111.006435 58 SiO2 215.231417 59 Nb2O5 110.245668 60 SiO2 214.169518 61 Nb2O5 111.027028 62 SiO2 214.34487 63 Nb2O5 111.673756 64 SiO2 214.244813 65 Nb2O5 111.079041 66 SiO2 213.789538 67 Nb2O5 112.533959 68 SiO2 214.210094 69 Nb2O5 112.724502 70 SiO2 214.10514 71 Nb2O5 111.816605 72 SiO2 214.903751 73 Nb2O5 111.375223 74 SiO2 216.600697 75 Nb2O5 108.409546 76 SiO2 217.075285 77 Nb2O5 105.631166 78 SiO2 219.15066 79 Nb2O5 104.675816 80 SiO2 221.358226 81 Nb2O5 100.306396 82 SiO2 221.855185 83 Nb2O5 96.432497 84 SiO2 219.84993 85 Nb2O5 101.481757 86 SiO2 217.432062 87 Nb2O5 108.404809 88 SiO2 213.890199 89 Nb2O5 111.04977 90 SiO2 217.589309 91 Nb2O5 106.544147 92 SiO2 114.923947

The design for this filter was produced by optimizing a standard dichroic filter comprising alternating quarter wavelength thick layers of SiO₂ and Nb₂O₅ against the target spectra shown in FIGS. 10 and 11.

The filter configuration was physically produced using a computer controlled ion-beam deposition system, such as the one described in U.S. Pat. No. 7,068,430. The resulting filter was measured at 45° Angle of incidence using a Perkin Elmer Lambda 900 spectrophotometer with a 2-degree cone-half-angle beam at the filter. A magnified portion of the measured spectra 131, 133, 135 is shown in FIG. 12. As shown, average spectrum 133 exhibited a non-harmonic second stopband region having long wave edge at about 536 nm. This edge had an edge steepness (10%-90% T) for light of average polarization of 0.65%, relative to the edge wavelength. Furthermore, polarization splitting at the 50% transmission point for this edge was 0.45%. The filter also exhibited a fundamental stopband having a short wave edge around 850 nm (not shown in FIG. 12). The passband bandwidth was 58.9%, relative to the long wave edge of the non-harmonic second stopband region.

Spectra 131, 133, and 135 shown in FIG. 12 did not precisely correlate to the design spectra shown in FIG. 11. To clarify this issue, simulated spectra 141, 143, 145 (shown in FIG. 13) of a 532 nm steep dichroic beamsplitter were plotted using a 2-degree cone-half-angle beam at the filter. The simulated spectra of FIG. 13 closely correlated to the measured spectra of FIG. 12. Thus, the simulated spectra demonstrated that the disagreement between the measured spectra in FIG. 12 and the design spectra in FIG. 11 was dominated by the non-collimated nature of the spectrophotometer beam.

An additional 532 nm steep dichroic beamsplitter (example 4) was produced largely in accordance with the design of example 3, but had a total coating thickness of about 14.3 μm. This filter was then compared to the filter of example 3, so as to investigate the impact of coating thickness on edge steepness. The data obtained is reproduced in the following table.

TABLE 4 Dependence of coating thickness on edge steepness for 532 nm dichroic beam splitters: Thickness CHA Edge Steepness (μm) (Deg) 1% 2% 5% 10% 50% 90% 1%-50% 2%-50% 5%-50% 10%-50% Ex. 3 14.6 0.00 531.77 532.45 533.20 533.70 535.32 537.16 0.67% 00.54% 0.40% 0.29% Ex. 4 14.3 0.00 530.24 531.47 532.60 533.28 534.85 535.72 0.87% 0.63% 0.42% 0.29%

As shown, as coating thickness is increased, the value of edge steepness decreased over all indicated transmission ranges. That is, the 532 nm filter having a coating thickness of about 14.6 μm exhibited better edge steepness than a filter having a similar design having a coating thickness of about 14.3 μm

Examples 5-7 785 nm Steep Dichroic Beamsplitter

A 785 nm dichroic beamsplitter optical filter design was produced having the configuration shown in the following table.

TABLE 5 structure of a 785 nm dichroic beamsplitter 785 nm Steep Dichroic Beamsplitter Coating Thickness (μm): 22.027679258 Total Layers: 102 Layer Material Thickness (nm) 1 Nb2O5 32.65508 2 SiO2 144.868199 3 Nb2O5 47.804313 4 SiO2 128.394549 5 Nb2O5 238.282792 6 SiO2 278.529226 7 Nb2O5 205.569093 8 SiO2 192.698961 9 Nb2O5 213.370004 10 SiO2 229.798324 11 Nb2O5 202.31804 12 SiO2 229.540528 13 Nb2O5 188.952969 14 SiO2 264.186477 15 Nb2O5 166.668797 16 SiO2 298.763611 17 Nb2O5 151.800937 18 SiO2 306.540671 19 Nb2O5 153.743939 20 SiO2 292.519538 21 Nb2O5 167.77654 22 SiO2 265.665013 23 Nb2O5 181.805452 24 SiO2 248.085954 25 Nb2O5 186.587916 26 SiO2 257.083035 27 Nb2O5 173.731249 28 SiO2 282.190763 29 Nb2O5 159.397771 30 SiO2 300.391953 31 Nb2O5 150.96526 32 SiO2 297.936495 33 Nb2O5 162.276647 34 SiO2 278.171719 35 Nb2O5 176.668889 36 SiO2 253.863045 37 Nb2O5 184.792068 38 SiO2 254.299535 39 Nb2O5 177.746861 40 SiO2 272.711789 41 Nb2O5 163.373578 42 SiO2 296.233849 43 Nb2O5 153.524699 44 SiO2 299.382853 45 Nb2O5 158.508489 46 SiO2 281.140846 47 Nb2O5 174.034337 48 SiO2 258.219413 49 Nb2O5 182.515237 50 SiO2 252.516009 51 Nb2O5 180.195999 52 SiO2 270.147819 53 Nb2O5 165.079108 54 SiO2 292.635319 55 Nb2O5 154.092521 56 SiO2 300.457145 57 Nb2O5 157.556848 58 SiO2 283.249826 59 Nb2O5 170.688922 60 SiO2 263.521222 61 Nb2O5 182.754251 62 SiO2 250.831796 63 Nb2O5 180.190883 64 SiO2 267.736417 65 Nb2O5 168.130035 66 SiO2 289.992505 67 Nb2O5 154.816559 68 SiO2 301.48422 69 Nb2O5 156.252091 70 SiO2 292.362158 71 Nb2O5 167.902499 72 SiO2 267.994834 73 Nb2O5 182.180612 74 SiO2 256.094057 75 Nb2O5 185.837901 76 SiO2 256.023623 77 Nb2O5 182.259064 78 SiO2 275.674377 79 Nb2O5 172.851921 80 SiO2 293.233127 81 Nb2O5 167.346137 82 SiO2 309.467491 83 Nb2O5 166.563984 84 SiO2 315.796872 85 Nb2O5 168.81202 86 SiO2 317.38589 87 Nb2O5 166.46106 88 SiO2 318.205316 89 Nb2O5 166.733122 90 SiO2 321.459683 91 Nb2O5 165.490765 92 SiO2 324.579443 93 Nb2O5 163.747759 94 SiO2 314.956531 95 Nb2O5 162.072937 96 SiO2 322.030717 97 Nb2O5 168.63477 98 SiO2 359.00252 99 Nb2O5 101.921796 100 SiO2 91.621751 101 Nb2O5 14.838899 102 SiO2 79.718824

Like examples 3 and 4 above, the design for the filter of example 5 was produced by optimizing a standard dichroic filter comprising alternating quarter wavelength thick layers of SiO₂ and Nb₂O₅. However, in this case, the basic structure and spectra were optimized against the target spectra 153, 161, 163, 165 shown in FIGS. 14 and 15.

The resulting filter exhibited spectral characteristics, when measured at 45° Angle of incidence, largely consistent with the target spectra. That is, this filter exhibited a fundamental stopband region having a short wave edge around 1240-1250 nm, and a non-harmonic second stopband region below about 780 nm. A bandpass region separated the fundamental stopband region and the non-harmonic second stopband region. The bandpass bandwidth was about 56%, relative to the long wave edge of the non-harmonic second stopband region. the edge steepness at the long wave edge of the non-harmonic stopband was 0.24%. the edge steepness (10%-90% T) at the long wave edge of the non-harmonic stopband was 0.52%.

Further, like examples 3 and 4, multiple filters in accordance with this design were designed having different overall coating thicknesses. The design spectra were compared and the resulting data is provided in the following table:

TABLE 6 Dependence of coating thickness on edge steepness for 785 nm dichroic beamsplitters: Thickness CHA Edge Steepness (μm) (Deg) 1% 2% 5% 10% 50% 90% 1%-50% 2%-50% 5%-50% 10%-50% Ex. 5 22.0 0.00 784.33 786.40 787.57 790.22 791.67 0.74% 0.48% 0.34% Ex. 6 21.5 0.00 783.33 785.65 786.91 789.51 790.85 0.79% 0.49% 0.33% Ex. 7 28.4 0.00 784.32 785.65 786.91 787.69 789.55 790.76 0.66% 0.50% 0.33% 0.24% 0.25 784.35 786.36 787.53 790.21 791.74 0.74% 0.49% 0.34% 0.50 784.24 786.24 787.41 790.16 791.93 0.75% 0.50% 0.35% 1.00 783.84 785.79 786.97 790.03 792.62 0.79% 0.54% 0.39%

Example 7 Deeply Blocking, Steep 532 nm Beamsplitter Design

A design for a deeply blocking, steep 532 nm beamsplitter for 45° Angle of incidence spectroscopy was designed having the configuration shown in the following table.

TABLE 7 design structure of a steep 532 nm beamsplitter for 45° Angle of incidence: 532 nm Deeply Blocking Steep Beamsplitter Coating Thickness (μm): 28.441678135 Total Layers: 240 Layer Material Thickness (nm) 1 Nb2O5 125.326398 2 SiO2 189.82584 3 Nb2O5 142.239469 4 SiO2 173.603106 5 Nb2O5 117.551329 6 SiO2 133.761606 7 Nb2O5 80.048395 8 SiO2 176.142504 9 Nb2O5 94.522896 10 SiO2 130.594691 11 Nb2O5 79.110445 12 SiO2 171.423982 13 Nb2O5 95.099021 14 SiO2 130.08483 15 Nb2O5 78.427458 16 SiO2 166.362706 17 Nb2O5 98.106453 18 SiO2 132.199068 19 Nb2O5 77.000752 20 SiO2 157.603991 21 Nb2O5 101.809349 22 SiO2 136.519889 23 Nb2O5 76.525829 24 SiO2 149.287636 25 Nb2O5 103.878664 26 SiO2 141.700953 27 Nb2O5 76.171685 28 SiO2 141.748625 29 Nb2O5 102.836029 30 SiO2 151.235606 31 Nb2O5 76.671595 32 SiO2 135.237111 33 Nb2O5 99.608324 34 SiO2 161.797651 35 Nb2O5 77.413062 36 SiO2 131.503309 37 Nb2O5 94.771406 38 SiO2 168.571161 39 Nb2O5 80.958326 40 SiO2 128.724034 41 Nb2O5 88.765883 42 SiO2 172.451601 43 Nb2O5 85.879418 44 SiO2 128.608638 45 Nb2O5 82.76891 46 SiO2 171.164311 47 Nb2O5 91.683182 48 SiO2 131.169369 49 Nb2O5 78.174113 50 SiO2 165.223996 51 Nb2O5 96.979355 52 SiO2 135.753497 53 Nb2O5 75.953529 54 SiO2 155.54757 55 Nb2O5 100.806536 56 SiO2 142.76079 57 Nb2O5 74.918393 58 SiO2 145.729079 59 Nb2O5 101.128401 60 SiO2 152.577714 61 Nb2O5 75.495573 62 SiO2 137.404314 63 Nb2O5 98.714134 64 SiO2 161.823972 65 Nb2O5 77.800849 66 SiO2 131.799092 67 Nb2O5 93.62446 68 SiO2 169.469091 69 Nb2O5 81.286423 70 SiO2 129.760412 71 Nb2O5 87.106824 72 SiO2 172.74435 73 Nb2O5 86.67709 74 SiO2 129.628565 75 Nb2O5 81.980207 76 SiO2 169.63066 77 Nb2O5 93.161138 78 SiO2 131.596083 79 Nb2O5 78.207451 80 SiO2 162.830265 81 Nb2O5 97.829036 82 SiO2 138.108686 83 Nb2O5 75.326723 84 SiO2 153.591247 85 Nb2O5 100.724734 86 SiO2 145.680217 87 Nb2O5 75.197663 88 SiO2 144.136845 89 Nb2O5 100.023055 90 SiO2 155.996046 91 Nb2O5 75.797013 92 SiO2 136.731146 93 Nb2O5 96.701027 94 SiO2 164.807958 95 Nb2O5 78.485102 96 SiO2 131.486595 97 Nb2O5 91.416386 98 SiO2 170.843571 99 Nb2O5 82.948453 100 SiO2 129.597724 101 Nb2O5 85.684493 102 SiO2 171.307995 103 Nb2O5 89.26102 104 SiO2 130.290103 105 Nb2O5 80.451223 106 SiO2 167.153794 107 Nb2O5 95.259769 108 SiO2 133.387034 109 Nb2O5 77.191911 110 SiO2 158.612514 111 Nb2O5 99.755764 112 SiO2 139.656216 113 Nb2O5 75.567149 114 SiO2 149.125107 115 Nb2O5 101.280773 116 SiO2 148.680223 117 Nb2O5 75.567766 118 SiO2 140.751704 119 Nb2O5 99.493004 120 SiO2 158.91319 121 Nb2O5 76.931062 122 SiO2 133.924406 123 Nb2O5 95.587146 124 SiO2 167.046455 125 Nb2O5 79.941891 126 SiO2 130.595713 127 Nb2O5 90.061043 128 SiO2 171.52332 129 Nb2O5 84.639663 130 SiO2 129.724731 131 Nb2O5 84.731083 132 SiO2 170.853256 133 Nb2O5 90.506855 134 SiO2 130.698407 135 Nb2O5 80.919633 136 SiO2 166.148719 137 Nb2O5 95.948475 138 SiO2 134.107378 139 Nb2O5 78.455937 140 SiO2 160.402121 141 Nb2O5 99.679602 142 SiO2 138.661918 143 Nb2O5 77.568835 144 SiO2 155.399004 145 Nb2O5 101.437398 146 SiO2 143.460187 147 Nb2O5 77.747731 148 SiO2 150.835494 149 Nb2O5 102.425545 150 SiO2 147.617068 151 Nb2O5 77.514841 152 SiO2 148.320077 153 Nb2O5 102.383455 154 SiO2 150.835187 155 Nb2O5 77.594384 156 SiO2 144.567254 157 Nb2O5 101.963371 158 SiO2 153.995372 159 Nb2O5 77.581 160 SiO2 139.575503 161 Nb2O5 100.629357 162 SiO2 158.270796 163 Nb2O5 78.23092 164 SiO2 134.943788 165 Nb2O5 97.362407 166 SiO2 164.518246 167 Nb2O5 79.948805 168 SiO2 131.020116 169 Nb2O5 92.463462 170 SiO2 169.687184 171 Nb2O5 83.612812 172 SiO2 129.311392 173 Nb2O5 86.405451 174 SiO2 171.732418 175 Nb2O5 88.583284 176 SiO2 129.57488 177 Nb2O5 81.449042 178 SiO2 168.473041 179 Nb2O5 94.322516 180 SiO2 131.891973 181 Nb2O5 77.64882 182 SiO2 160.980285 183 Nb2O5 99.082782 184 SiO2 136.972706 185 Nb2O5 76.270067 186 SiO2 150.794779 187 Nb2O5 101.82662 188 SiO2 144.810218 189 Nb2O5 76.247458 190 SiO2 142.197031 191 Nb2O5 101.08276 192 SiO2 154.472747 193 Nb2O5 76.645516 194 SiO2 135.588916 195 Nb2O5 97.089812 196 SiO2 164.201697 197 Nb2O5 78.884226 198 SiO2 130.455866 199 Nb2O5 92.066688 200 SiO2 170.656157 201 Nb2O5 82.645702 202 SiO2 129.356456 203 Nb2O5 86.614591 204 SiO2 172.151688 205 Nb2O5 88.107814 206 SiO2 129.509539 207 Nb2O5 81.813237 208 SiO2 168.307689 209 Nb2O5 94.040778 210 SiO2 131.741612 211 Nb2O5 78.477373 212 SiO2 161.070545 213 Nb2O5 99.303611 214 SiO2 135.909174 215 Nb2O5 76.189676 216 SiO2 153.895605 217 Nb2O5 101.524691 218 SiO2 142.414629 219 Nb2O5 75.918059 220 SiO2 143.004075 221 Nb2O5 104.234156 222 SiO2 148.478627 223 Nb2O5 76.261156 224 SiO2 136.688637 225 Nb2O5 104.346797 226 SiO2 156.347975 227 Nb2O5 75.50798 228 SiO2 131.274802 229 Nb2O5 106.072884 230 SiO2 161.095229 231 Nb2O5 73.862257 232 SiO2 123.798118 233 Nb2O5 120.45028 234 SiO2 139.021448 235 Nb2O5 61.510898 236 SiO2 186.49687 237 Nb2O5 110.855316 238 SiO2 95.411626 239 Nb2O5 112.998509 240 SiO2 85.040064

Like the above examples, the design for the filter of example 8 was produced by optimizing a standard dichroic filter comprising alternating quarter wavelength thick layers of SiO₂ and Nb₂O₅. However, in this case, the basic structure and spectra were optimized against the target spectra 173, 181, 183, and 185 shown in FIGS. 16 and 17.

The calculated spectrum of this design exhibited a fundamental stopband region having a short wave edge around 690 nm and a long wave edge around 850 nm. The calculated spectrum also exhibited a non-harmonic stopband having a long wave edge around 540 nm, and a shortwave edge around 520 nm. A passband region separated the fundamental and non-harmonic stopband regions, and had a passband bandwidth of 23.5%, relative to the long wave edge of the non-harmonic stopband. The edge steepness (10-90% T of the long wave edge of the non-harmonic stopband was 0.10%. Finally, the calculated spectrum shows that the design substantially blocks 100% of light (OD 6 or greater) having a wavelength within the fundamental and non-harmonic stopband regions.

Other embodiments of the invention will be apparent to those skilled in the art from consideration of the specification and practice of the invention disclosed herein. It is intended that the specification and examples be considered as exemplary only, with a true scope and spirit of the invention being indicated by the following claims. 

What is claimed is:
 1. An interference filter, comprising: a substrate, and a plurality of alternating first and second material layers deposited on said substrate, said material layers having respectively different refractive indices; wherein said plurality of alternating first and second material layers are configured such that when light from a light source impinges on said filter at an Angle of incidence of about 45°, said filter defines a spectrum for light of average polarization, said spectrum comprising a first stopband region and a second stopband region separated by a passband region having a passband bandwidth; wherein said first stopband region corresponds to a fundamental stopband; wherein said second stopband region comprises a long wave edge and a short wave edge, and encompasses a range of wavelengths other than wavelengths corresponding to a harmonic of the first stopband region; and wherein at least one of said long wave edge and said short wave edge has an edge steepness, when measured between 10-90% transmission, of about 0.75% or less, relative to an edge wavelength of said second stopband region.
 2. The interference filter of claim 1, wherein at least one of said long wave edge and short wave edge exhibits an edge steepness, when measured from optical density 3 to 90% transmission, of about 0.50% or less.
 3. The interference filter of claim 1, wherein at least one of said long wave edge and short wave edge exhibits an edge steepness, when measured from optical density 1 to 90% transmission, of about 0.25% or less. 